Method of stopping-off plating in electroplating baths

ABSTRACT

A PROCESS FOR ELECTROPLATING A PLATE METAL FROM A BATH ONTO A CATHODE HAVING PREDETERMINED AREAS TO BE PLATED AND SELECTED AREAS TO BE STOPPED OFF WHICH COMPRISES PLACING AN AUXILIARY ELECTRODE ADJACENT TO SAID SELECTED AREAS, MAINTAINING SAID SELECTED AREAS ANODIC TO SAID AUXILIARY ELECTRODE AND SUBJECTING SAID CATHODE TO PLATING CURRENT DENSITY WHEREBY SAID METAL IS DEPOSITED ON THE SAID PREDETERMINED AREAS AND SAID SELECTED AREAS REMAIN STOPPED OFF, AND AN APPARATUS FOR ELECTROPLATING WHICH COMPRISES AN ELECTROPLATING BATH, AT LEAST ONE ANODE THEREIN, A CATHODE HAVING PREDETERMINED AREAS TO BE PLATED AND SELECTED AREAS TO BE STOPPED, OFF AN AUXILIARY ELECTRODE ADJACENT TO EACH OF THE SAID SELECTED AREAS OF SAID CATHODE, A MAIN POWER SUPPLY ESTABLISHING A POTENTIAL ACROSS THE ANODE AND THE CATHODE, AND MEANS FOR MAINTAINING SAID SELECTED AREAS ANODIC TO SAID AUXILIARY ELECTRODE. IN ONE EMBODIMENT, THE MEANS FOR MAINTAINING SAID SELECTEDAREAS ANODIC TO SAID AUXIALIARY ELECTRODE INCLUDES A POLARIZING POWER SUPPLY ESTABLISHING A POTENTIAL BETWEEN SELECTED AREAS OF THE CATHODE AND THE AUXILIARY ELECTRODE, THE POSITIVE TERMINAL OF THE POLARIZING POWER SUPPLY BEING JOINED TO THE NEGATIVE TERMINAL OF THE MAIN POWER SUPPLY. THE AUXILIARY ELECTRODE MAY CONTAIN A THIN COATING OF PLATINUM SUFFICIENT TO SUBSTANTIALLY INHIBIT ELECTROPLATING OF SAID AUXILIARY ELECTRODE.

R. D. BEDI March 30-, 1971 METHOD OF STOPPING-OFF PLATING INELECTROPLATING BATHS Original Filed July 1, 1963 INVENTOR RAM DEV 550/3,573,175 METHOD OF STQPPlN-G-OFF PLATING IN ELECTROPLATING BATHS RamDev Bedi, Oak Park, Mich., assignor to M & T Chemicals Inc., New York,NY. Continuation of application Ser. No. 291,987, July 1, 1963. Thisapplication May 1, 1968, Ser. No. 725,956 Int. Cl. C23b 5/48; B01k 3/02US. Cl. 204-15 2 Claims ABSTRACT OF THE DISCLOSURE A process forelectroplating a plate metal from a bath onto a cathode havingpredetermined areas to be plated and selected areas to be stopped offwhich comprises placing an auxiliary electrode adjacent to said selectedareas, maintaining said selected areas anodic to said auxiliaryelectrode and subjecting said cathode to plating current density wherebysaid metal is deposited on the said predetermined areas and saidselected areas remain stopped off, and an apparatus for electroplatingwhich comprises an electroplating bath, at least one anode therein, acathode having predetermined areas to be plated and selected areas to bestopped off, an auxiliary electrode adjacent to each of the saidselected areas of said cathode, a main power supply establishing apotential across the anode and the cathode, and means for maintainingsaid selected areas anodic to said auxiliary electrode. In oneembodiment, the means for maintaining said selected areas anodic to saidauxiliary electrode includes a polarizing power supply establishing apotential between selected areas of the cathode and the auxiliaryelectrode, the positive terminal of the polarizing power supply beingjoined to the negative terminal of the main power supply. The auxiliaryelectrode may contain a thin coating of platinum suificient tosubstantially inhibit electroplating of said auxiliary electrode.

This application is a continuation of application S.N. 291,987, filedJuly 1, 1963, now abandoned.

This invention relates to a method of electroplating and moreparticularly to a method of stopping-off selected areas of electrodes topermit control of these selected areas so that they may be free fromplate which is to be deposited on other areas of the electrode.

As is well known to those skilled in the art, metals may be immersed invarious baths in the course of different treating processes wherein adesired portion of the metal may, for example, be subjected toelectroplating. Electroplating of various metals, typically nickel,copper, tin, zinc, or chromium may be effected in baths of varyingacidity and composition; many of these baths may contain chloride orfluoride which may be an integral part of the bath or present as anadditive. Because of differences in area or geometry or accessibility,there may be predetermined portions of the cathode which are to beplated and selected areas on Which a plate is not desired. The surfaceof these selected areas, if not properly treated to prevent platingthereon, i.e. if not stopped-off, may undesirably be plated suflicientlyto alter the dimensions substantially and/or to spoil the appearance ofthe finished article. The problem of stop-off may be present in variousbaths, typified by chromium plating baths, nickel plating baths, acidcopper baths, acid tin baths, zinc baths containing halides orhalide-complexes, etc.; for purpose of convenience, reference willhereinafter be made to chromium plating baths.

Commonly the areas of the cathode on which plate is not desired may becovered or painted with a masking or stop-off material which preventsdeposition and build- United States Patent 0 ICC.

up of the plate on these high current density areas which would normallybe plated. The masking or stop-off coatings are commonly subjected tosevere corrosive conditions for an extended period of time. For example,in chormium plating these materials may remain in position for extendedperiods of time, often for days, under the most severe oxidizingconditions in contact with baths which may contain sulfuric acid andchromic acid together with numerous other chemicals. Furthermore thestop-off material may be subjected to the highly erosive effect of thehydrogen which is liberated.

As is well known to those skilled in the art, chromium plating forexample may be effected by use of a bath containing chromic acid andsulfate together with other compounds Which may be employed to effectvarious desirable results. Typical of these additive compounds may befluorides or fluoride complexes. Other illustrative chromium platingsystems may include soluble catalyst systems containing e.g. chromicacid and sulfate such as sulfuric acid in amount sufficient to give thedesired concentration of sulfate ion, or self-regulating baths,typically those containing silicofluorides together with sulfate.

During chromium plating from baths typified by the foregoing, it iscommon to operate at a temperature which may vary depending upon thetype and purpose of the plating operation. Commonly however thetemperature of operation may be 34 C.72 C. and typically 48 C.63 C. Thecathodic current density may preferably be controlled to fall in therange of 8-100, and typically 12-50 a.s.d. (i.e. amperes per squaredecimeter) on the selected areas of high current density whereon platingmay occur. However, because there may be certain selected high currentdensity areas which are not to be chromium plated, it is desired tostop-off these areas.

In chromium plating systems, it has heretofore been common to coat withtapes or waxes those selected portions of the cathode where no plate isdesired. The use of such techniques is time consuming and expensive.Tapes or waxes must be carefully placed and removed, since any holes orspaces (including edge areas which may be undermined) in the protectivecoating will allow plating of the so-exposed metal to take place. Suchtapes or waxes may be expensive to purchase and apply. Various otherattempts to stop-off selected areas of cathodes in the noted platingbaths have not been uniformly successful, and there is today noeconomical, commercial stop-off system which is completely satisfactoryfrom the production point of view.

Various other processes are also known, including chemical treating,etching of selected portions of a piece, anodizing of pieces, etc. whereit is desired to protect certain portions of the workpiece from theaction of the solution. Stop-off materials to effect these results haveheretofore been either paints or tapes. Typical paints may includeplastic materials such as polymers of vinyl chloride or vinyl acetatewhich may contain plasticizers, pigments, solvents, etc. These may beapplied by painting onto the desired portions of the workpiece which arenot to be subjected to the action of the solution. Masking tapes whichmay be employed for the same purpose may be wrapped around the piece andcover the areas which are to be protected or stopped-off.

As is well known to those skilled in the art, application of e.g.masking tapes and paints is a time consuming and expensive job.Furthermore, many of the stopoff paints or tapes are unsatisfactory inthat, during plating, they may become pervious to the solution or not adherent to the workpiece. Perviousness permits growth of dendritic treesat or adjacent to the tape or paint; such growths must be removed bygrinding or machining to make the part satisfactory with respect toappearance and dimensionality. It is a particular objection that tapesand paints permit build-up of the plate at the point immediatelyadjacent to the non-conducting tape or paint and thus may require agrinding operation, after plating is complete, to remove the ridgeswhich may be present. It is a further objection that paint or maskingtape must be removed from the workpiece at the completion of the platingoperation. This may typically be done by cutting, peeling, scraping, byaction of solvents, etc.

It is an object of this invention to provide a novel technique formasking or stopping-off areas to be maintained free of a deposited plateduring plating. Other objects will be apparent to those skilled in theart on inspection of the following detailed description.

In accordance with certain of its aspects, the process of this inventionfor electroplating a plate metal from a bath onto a cathode havingpredetermined areas to be plated and selected areas to be stopped-offcomprises placing an auxiliary electrode adjacent to said selectedareas, maintaining said selected areas anodic to said auxiliaryelectrode, and subjecting said cathode to plating current densitywhereby said plate metal is deposited on the said predetermined areasand said selected areas remain stopped-off.

In practice of this invention, the selected areas to be stopped-oft maybe stopped-off by placing adjacent thereto an auxiliary electrode. Themetal from which the auxiliary electrode may be fabricated may be anyconvenient or available metal typically iron, steel, etc. Preferably theauxiliary electrode may be formed of a metal which possesses a highdegree of resistance to the plating bath under the conditions ofoperation. In a preferred embodiment the auxiliary electrode may bestopped-oil by any conductive means which will minimize or eliminatetrodeposition thereon of the metal being plated, while simultaneouslypermitting current flow thereto.

In a preferred embodiment of this invention, the auxiliary electrode maybe stopped-cit by means of the technique disclosed in applicantscopending patent application. Typically such stopping-01f of theauxiliary electrode may be efitected by immersing the auxiliaryelectrode in a solution of e.g. platinum chloride, sodium chloride, andhydrochloric acid sufiicient to lower the pH to about 1.5, whereby theauxiliary electrode may become immersion plated with a thin coating ofplatinum. This thin coating of platinum may be sufiicient to eliminateelectroplating of metal e.g. chromium on the auxiliary electrode duringpractice of the instant invention, but it will not interfere with theflow of current from the auxiliary electrode.

In practice of the invention, the auxiliary electrode, preferablytreated as hereinbefore disclosed, may be placed within theelectroplating solution adjacent to the selected areas whereon platingis not desired. Preferably the auxiliary electrode will be of size,shape, and configuration to substantially cover the entire selectedarea. Typically the auxiliary electrode will conform to the saidselected areas. Where the selected areas are spaced from each other, theauxiliary electrode may be composed of several independent auxiliaryelectrode pieces elec trically connected to each other and to theauxiliary electrode circuit as hereinafter described in detail.Preferably the auxiliary electrode(s) will be closely adjacent to thesaid selected areas. Typically the auxiliary electrode may be spacedfrom the selected areas by a distance of 1 mm. to 10 mm., say 5 mm. Itwill be apparent to those skilled in the art that the particulardistance will depend upon various considerations including the size ofthe selected area and of the auxiliary electrode. Thus where theselected area and auxiliary electrode are both small, it may be possibleto attain the desired results by positioning the auxiliary electrodeclose to the selected area. Where the selected area is quite extensive,it may be desirable to space the auxiliary electrode a greater distancetherefrom.

When electroplating according to this invention, the cathode piece maybe placed within an appropriate electrolyte bath containing one or moreanodes. The anodes and cathodes may be energized by a direct currentpower supply. In practice of the invention, the auxiliary electrode maybe energized by a polarizing power supply, the positive leg of which maybe connected to the negative leg of the main power supply for the anodeand cathode. The negative leg of the polarizing power supply may beconnected to the auxiliary electrodes. The preferred polarizing powersupply which may be employed in practice of this invention may be onecharacterized by a high internal resistance.

The energy output of the polarizing power supply will be such, withrespect to the main power supply, that the selected areas of the cathodeare maintained anodic to the auxiliary electrodes. Typically this may beeffected, egg. in the case of chromium plating, by maintaining apotential difference between the selected areas of the cathode and theauxiliary electrode of at least 2 volts, and preferably 3 to 5 volts inthe case of chromium plating. In the case of nickel plating, thisvoltage may be eg. 0.20.3 volt.

Thus a preferred embodiment of the apparatus by which the instantinvention may be practiced may include, suspended in an electroplatingbath, an anode (or anodes), a cathode having predetermined areas to beplated and selected areas to be stopped-01f, an auxiliary electrode (orelectrodes) suspended adjacent to each of the said selected areas ofsaid cathode, a main power supply establishing a potential across theanode and the cathode, and a polarizing power supply establishing apotential between selected areas of the cathode and the auxiliaryelectrode, the positive terminal of the polarizing power supply beingjoined to the negative terminal of the main power supply.

Electroplating by the process of this invention may thus be effected bymaintaining the anode, cathode, a auxiliary electrode in the noted bathfor a predetermined time at desired conditions of temperature, currentdensity, etc. During this time, the cathode may be electroplated and itwill be found that the selected areas to be stoppedoff in fact havereceived no deposit of plate.

In an illustrative embodiment of this invention as shown in the attacheddrawing, a plating solution 10 may be contained in vessel 11 whichcontains a cylindrical cathodic piece generally designated 12. Thecathode piece 12 may contain predetermined areas 13 on the outer surfacethereof which areas are to be electroplated. The cathode 12 may alsocontain selected areas 14 whereon plating is not desired. Solution 10also contains therewithin anode(s) 15. Cathode 12 and anode(s) 15 may beelectrically connected to power supply 16 through conductors 17 and 18.

This embodiment of the apparatus also includes auxiliary electrodes 19which may be thin cylindrical rings spaced from selected areas 14, whichin the instant embodiment may be threads on a pipe if the cathode 12 bea threaded pipe or rod. Auxiliary electrodes 19 may be connected topolarizing power supply 20 through conductor 21 which joins auxiliaryelectrodes 19 to the negative terminal of power supply 20. The positiveterminal of polarizing power supply 26 may be joined to the negativeterminal of power supply 16 through conductor 22. The polarizing powersupply circuit may include voltmeter 23.

Main power supply 16 may for example be an appropriate direct currentgenerator. The polarizing power supply 20 may preferably be onecharacterized by high internal impedance. Preferably the relationshipbetween power supply 16 and polarizing power supply 20 may be such thatduring operation of the process, the voltage difierential betweenselected areas 14 of cathode 12 and auxiliary electrode 19 may e.g. inthe case of a chromium plating bath typically be at least about 2 volts,and typically 3 to 5 volts.

In one illustrative example using the apparatus of the drawing, cathode12 may be a threaded steel pipe 19 cm.

long and 3.9 cm. outside diameter. The threaded areas 14 wereapproximately 2.5 cm. wide. The auxiliary electrodes 19 each werecylindrical pieces of Type 1040 steel having an internal diameter of 4.3cm. and an external diameter of 4.6 cm. The height (i.e. width) of eachauxiliary electrode was 2.5 cm.i.e., approximately the same as the widthof the threads of selected areas 14 of cathode 12. The auxiliaryelectrodes 19 were each mounted on an appropriate support therebymaintaining them in position.

In practice of a specific embodiment of the process of this invention,the electrolyte bath may (for use in chromium plating) contain 225 g./l.chromic acid and 2.25 g./l. of sulfate ion from sulfuric acid. Anodesmay be a lead-tin alloy and the cathode 12 may be a low carbon steel.The auxiliary electrodes 19 were of Type 1040 steel. The bath wasmaintained at 50 C. Electroplating was effected at current density of 31a.s.d. for 60 minutes. The auxiliary electrodes 19 were maintained at apotential of 3 volts with respect to the selected areas 14 of thecathode, and 15 amperes of current passed between the auxiliaryelectrodes 19 and the selected areas 14.

After 60 minutes, it was found that the predetermined areas 13 hadreceived a bright lustrous chromium deposit having a thickness of 30microns. The selected areas 14 were found to be entirely free of anychromium deposit and were entirely free of any change.

In another illustrative embodiment of this invention, nickel plating maybe effected in the apparatus of the drawing. The cathode 12 and theauxiliary electrodes 19 were identical to those set forth in the firstillustrative embodiment supra. The anodes 15 were of nickel. Theelectrolyte bath 10 may be a Watts nickel bath having the followingcomposition:

Component: Amount g./l. Nickel chloride 40 Nickel sulfate 300 Boric acid35 Electroplating may be effected at current density of 2 a.s.d. for 30minutes. The auxiliary electrodes 19 may be maintained at a potential of0.2-0.3 volt cathodic with respect to the selected areas 14 of thecathode, and 2 amperes current may pass between the auxiliary electrodes19 and the selected areas 14. After 30 minutes, it was found that thepredetermined areas had received a good Watts nickel deposit, having athickness of 16 microns. The selected areas 14 were found to be entirelyfree of any nickel deposit.

It is a particular feature of this invention that it permits attainmentof a highly uniform deposit of e.g. chromium from one end of the plateto the other end of the plate. The deposited plate is entirely free fromany bead effect and is characterized by its uniformly smooth surface andparticularly by the smooth transition between the plated area and theunplated area. This eliminates the need for any mechanical finishingafter plating.

As many embodiments of this invention may be made without departing fromthe spirit and scope thereof, it is to be understood that the inventionincludes all such modifications and variations as come within the scopeof the appended claims.

What is claimed is:

1. The process for electroplating a plate metal from a bath onto acathode having predetermined areas to be plated and selected areas to bestopped off which comprises placing an auxiliary electrode adjacent tosaid selected areas, said auxiliary electrode containing a thin coatingof platinum sufficient to substantially inhibit electroplating of saidauxiliary electrode, maintaining said selected areas anodic to saidauxiliary electrode, and subjecting said cathode to plating currentdensity whereby said metal is deposited on said predetermined areas andsaid selected areas remain stopped off.

2. Apparatus for electroplating which comprises a container for anelectroplating bath, at least one anode suspended in said container, anauxiliary electrode adjacent to said anode, said auxiliary electrodecontaining a thin coating of platinum sufiicient to substantiallyinhibit electroplating of said auxiliary electrode, a main power supplymeans for establishing a potential between said anode and a cathodesuspended in said container, and means for maintaining selected areas ofsaid cathode anodic to said auxiliary electrode.

References Cited UNITED STATES PATENTS 2,044,431 6/1936 Harrison 204-153,249,520 5/1966 Hermann 204-15 FOREIGN PATENTS 335,268 2/1936 Italy204-231 TA-HSUNG TUNG, Primary Examiner T. TULANIELLO, AssistantExaminer US. Cl. X.R. 204-231

